putting silicone on top of grout around bathtub

If there exists regular garden variety grout then it can be infiltrated by moisture. It must be able to breathe for any moisture to evaporate. Closing off its ability to allow for evaporation will likely grow mold under any caulk used.

Typically in junctures and any changes of plain caulk should be used, not grout. You could remove the existing grout and replace it with caulk.